1. Field of the Invention
The present invention relates to a link-member swinging apparatus which includes an actuator for swinging a link member.
2. Related Art Statement
There is known a reciprocating apparatus which is employed in each of various sorts of mechanical drive systems and which reciprocates a drive lever or a drive shaft belonging to the each drive system. For example, FIG. 12 shows a known reciprocating apparatus 110 which includes a stepper motor 112 as an electric actuator; a drive gear 114 fixed to an output shaft of the motor 112; a sector gear 118 meshed with the drive gear 114; and a drive shaft 116 which belongs to a mechanical drive system and which is fixed to the sector gear 118, or a drive lever 119 which belongs to a mechanical drive system and which is pivotally connected to one end portion of the sector gear 118. The stepper motor 112 is first rotated clockwise, subsequently is stopped for a very short time, then is rotated counterclockwise, and is stopped again for a very short time, and those actions are repeated. Thus, the stepper motor 112 reciprocates the drive lever 119 or the drive shaft 116 via the sector gear 118.
In the case where the mechanical drive system to which the drive lever 119 or the drive shaft 106 belongs has a great mass, i.e., a great inertia, when the sector gear 118 is stopped after being rotated, e.g., clockwise by the stepper motor 112, the stepper motor 112 is subjected to impact or vibration due to the great inertia of the drive system via the sector gear 118. In particular, in the case where the drive lever 119 of the reciprocating apparatus 110 laterally reciprocates, e.g., a needle bar of a zigzag sewing machine, at a high speed, the stepper motor 112 is subjected to a great impact or a large vibration due to the great inertia of the drive system including the needle bar. This leads to unstable stitch positions where a sewing needle attached to the needle bar penetrates a work-sheet such as a febric or a leather, or unstable sewing widths over which the needle is laterally reciprocated. Thus, irregular zigzag stitches are formed on the work-sheet. In addition, when the stepper motor 112 is rotated in the opposite direction a very short time after being stopped, the motor 112 may loose its synchronism.
The above problems may be solved by keeping the stepper motor 112 stopped for a long time sufficient for the vibration thereof resulting from the stopping thereof to dissipate substantially completely. In this case, however, the speed of lateral reciprocation of the needle bar cannot, be increased.
Meanwhile, it has been practiced to minimize the vibration of the stepper motor 112 at the time of being stopped, by applying permanently a certain frictional resistance to the mechanical drive system, or applying a certain additional load to reduce the vibration of the motor 112. In either case, however, the stepper motor 112 must be a larger-size one which can produce an additional torque to bear the frictional resistance or the additional load. This leads to increasing the production cost of the zigzag sewing machine.